Hi Brenda, The Otto Frei bench in the attached image (a wonderful
bench by the way) shows two arm rests, one on either side, under the
bench top. They are just long (about 20") smooth, rectangular boards
that pull out as desired. There are two L-shaped wooden brackets for
each arm rest, one facing either direction, screwed to the underside
of the top to create the slot for the arm rest. It's a pretty simple
design, which is often the best design.
I really don't use my arm rests very often, but it is nice to be
able to rest your elbow when you are preforming a task that requires
your arm to be elevated for an extended period of time.
Another nice feature to consider is a "bread board" that pulls out
above the drawers. It's handy to be able to pull it out to create
work space for a quick sketch or a place to put your ham and cheese
sandwich when the rest of your bench is covered with other stuff!
Hope that helps,